Lamda Phage Lysis and Lysogeny, Lytic Path, Genetic Regulation, Lysogenic Pathways, Feature of Lambda, Lytic Growth

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Lambda phage (lysis and lysogeny)

The phage has a genome consisting of 50kbp circular DNA. It follows either a lytic path (large number of phage particles are released due to lysis) or a lysogenic path (phage genome integrates with host's genome due to expression of gene cI and then replicates with host's genome).

Lytic path may be switched towards lysogeny and vice versa. For an account of genetic regulation of lytic and lysogenic pathways, readers are advised to consult author's text book Genetics.

An important feature of lambda () genome is that only about 60% of the genome (including left arm consisting of genes A-J measuring 20kb and the right arm, containing genes O-R and measuring 8-l0kb in length) is essential for lytic growth of the phage and the central portion (40%) can be replaced by foreign DNA.

The viability of phages goes down drastically, when its genome size is outside the acceptable limit of 78%-105% of the wild genome. This fixes the upper as well as the lower limits of size of foreign DNA which can be cloned.

A single break in the line phage DNA will generate two fragments, which are later joined together with foreign DNA to generate a chimeric phage particle. The chimeric phage can be multiplied after allowing it to infect bacteria and collecting progeny particles after a lytic cycle.

The use of phage particles as vector imposes a limitation on the size of foreign DNA, which can be cloned, because the capacity of phage head is only limited, and if the size of foreign DNA is too long, size of phage DNA may not be accommodated in phage head.

Lytic and Lysogenic paths in the life cycle of lambda (λ) bacteriophage

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Inorder to overcome this problem, those segments of DNA, which do not contain essential genes, may be removed. Such a technique has been followed in phage lambda () to create a smaller vector genome having single restriction site for the enzyme EcoRI.

Since the reduced size also fails to be adequately packed in phage head (there is also a requirement of a minimum size of DNA), this automatically provides a selection method, in which only the chimeric particles will be obtained in the phage progeny, and the vector particles lacking a cloned segment will be eliminated due to their reduced size.

Cloning of Foreign DNA

Cloning of Foreign DNA in a nonessential region of phage vector

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